*{
	margin: 0;
	padding: 0;
}

#main{
	position: absolute;/*main也要absolute，其子元素才会相对于它*/
	height: 200px;
	width: 400px;
	top:50%;
	left: 50%;
	margin-left: -200px;
	margin-top: -100px;
	overflow: hidden;
}

#left, #right{
	width: 100px;
	height: 100px;
	border-radius: 50px;
}

#left{
	position: absolute;
	top:-50px;
	left: -50px;
}

#right{
	position:absolute;
	bottom: -50px;
	right:-50px;
}